Some people in a general sense are sluts. This is an old assumption that bi and gay folk are more sexually careless that hasn't really been true since the aids crisis.

Fun fact, wherever there is stigma or barriers in seeking health care or populations that are discriminated against you see higher rates of STIs because people wait longer to check on things that go wrong and don't catch them before they take hold. This includes immigrant populations, religious minorities in hostile cultures, queer people, racial minorities, homeless populations... They aren't all rawdogging it, these groups often feel they can't be honest with a doc or fear being rejected from healthcare situations. Some queer folk have trauma around medical care in the past. Where management and early testing drops off disease transmission becomes more common.

You will see old studies positing the multiple partners thing but the cutting edge data has seen this is a problem faced by multiple populations with the core of the problem sourced and traceable to the degree of stigma against the patients.

* literal propaganda from the Aids epidemic